Bug#851526: Please provide command-line option to disable ipv6

[Josh Triplett <josh@joshtriplett.org>]

Package: netcfg
Severity: wishlist

netcfg provides an option to completely disable all automatic
configuration, but no option to disable ipv6 autoconfig (SLAAC) while
leaving DHCP enabled.  Putting ipv6.disable=1 on the kernel command line
will cause netcfg to realize the network has no ipv6, but only after
waiting a similar timeout for a link-local address, defeating the
purpose.

Please either detect disabled ipv6 and skip those steps, or provide a
command-line option to disable ipv6 in netcfg.

(Context: repeatedly testing preseed installs in a virtual machine, and
I don't want to keep waiting on ipv6 autoconfig timing out.)